The invention relates to a tool, especially a cutting insert, for the machining of metallic workpieces, composed of a hard metal base body, a cermet base body, a ceramic base body or a steel base body and at least one layer deposited thereon, in which the sole layer or in the case of a plurality of layers, the outermost layer or the layer lying beneath the outermost layer is constituted predominantly of molybdenum sulfide.
The invention also relates to a method of producing the molybdenum sulfide layer by means of CVD (chemical vapor deposition).
The coating of substrate bodies of hard metal, a cermet, a ceramic, or steel with hard materials or hard material mixtures or ceramic materials is known from the state of the art. Especially, carbides, nitrides, carbonitrides, oxycarbonitrides and/or borides of at least one of the elements of groups IVa to VIa of the Periodic Table are used, such as, for example, TiC, TiN, Ti(C,N) or also ceramic materials like Al2O3 or ZrO2. With large cutting speeds and/or large chip cross sections, high temperatures arise at the cutting edge of the machining tool especially in the case of materials which are difficult to machine, thereby giving rise to increased wear which can lead in part also to edge breakage. To minimize the temperatures which arise at the cutting edge of the tool during machining, usually so-called cooling lubricants have been used which contain either toxic and environmentally polluting substances and which must be disposed of at relatively high cost.
A further possibility, which can reduce the use of coolant lubricants drastically, is the principle of so-called minimum quantity lubrication, whereby very small amounts of coolant lubricants are supplied with point precision through fine passages to the location between the tool cutter and the workpiece. Apart from a high cost for carrying out the method and for the apparatus involved, this approach does not eliminate the waste treatment problem but only minimizes it.
On the other hand, there are so-called solid dry lubricant films, for example, molybdenum disulfide, known in the art. For example DE-A- 24 15 255 proposes to improve the adhesion of a layer which is preferably based upon MoS2 or WS2 and which is applied by so-called sputtering with a thickness of 1 to 2 xcexcm and in which the application of a dry lubricant layer based also upon MoS2 or WS2 can be effected in a conventional manner by a polishing application. In the East German patent document (DD) 202 898 it has been proposed to apply layers or the like of molybdenum disulfide by sputtering to cutting tools, stamping tools, drawing tools and machining tools, whereby the layer thickness lies in the nm range. The layer which is applied by sputtering to a substrate body provided with a hard material layer should have a hexagonal lattice structure. EP 0 534 905 A2 also proposes the PVD (plasma vapor deposition) coating of machining tools, inter alia with molybdenum disulfide.
It has, however, been found that MoS2 layers produced by cathodic atomization (sputtering) adhere poorly to the substrate body or to the outer hard material layer applied to the substrate body or have a predominantly unsatisfactory orientation of the MoS2 crystallites. To obtain the lowest friction coefficients, the hexagonal components of the MoS2 layer should be so oriented that the hexagonal base planes lie parallel to the surface. Thereby the oxidation resistance of the layer is also improved.
It is thus proposed in DE 35 16 933 A1 that the ratio of the water vapor partial pressure to the coating rate be adjusted to be smaller than 10xe2x88x927 mbar/1.6 mgmxe2x88x922sxe2x88x921 which, in practice, can only be achieved with considerable technological cost.
Apart from the requirement which is customary for the PVD process for purity, i.e. that the gas atmosphere not contain foreign substances, there is a further drawback with the PVD process in that a directed particle stream is provided from the target source to the substrate whereby the substrate must be rotated about three axes to produce a uniform coating.
Furthermore WO 97/04142 describes a process for uniform coating of MoS2 and a metal or a metal compound like titanium or titanium nitride through the use of two targets by means of a sputtering process.
While MoS2 is relatively soft and is not especially wear-resistant, the formation of an MoS2 coating has a significant effect since by the use of a single MoS2 layer or an outermost MoS2 layer at the contact location of the tool with the workpiece, the MoS2 may be quickly abraded but the friction force because of the penetration of the MoS2 particles from the edge inwardly (entrainment) is still noticeably reduced for a period of time.
It is the object of the present invention to provide a single layer or an outermost layer or layer combination which has a dense lattice structure and a higher hardness of greater mechanical stability against wear. It is also an object to provide a method of producing such a layer.
These objects are achieved by a tool in which the molybdenum sulfide layer contains a mixture of the sulfide phases MoS2 and MoS2S3 with a substantially random orientation of the phase crystals. The layered lattice structure is comprised of randomly oriented MoS2 grains which partly have a platelet-shaped configuration. The intervening spaces are filled with the Mo2S3 phase. With these features a surprisingly improved wear resistance of the layer is achieved on machining.
Thus still further phases of molybdenum sulfur compounds and/or chlorine can be contained in the molybdenum sulfide layer but should not exceed 6 atomic %.
A single molybdenum sulfide layer or the outermost molybdenum sulfide layer can also be fabricated so as to be of a multilayer type, whereby the respective undermost layer or an underlying layer with a thickness respectively not exceeding 200 nm has a hexagonal lattice structure with a preferred orientation of the hexagonal planes parallel to the tool surface. The remaining layers however are of isotropic orientation. This applies especially for a substrate body which is coated with only a single molybdenum sulfide layer (and no other layer). The layer with the hexagonal MoS2 crystals is thereby bounded directly by the substrate and has a thickness of 100 nm to 200 nm.
According to a further feature of the invention, the Mo2S3 component in the molybdenum layer does not remain continuously homogeneous but with increasing distance from the substrate body surface is present in different proportions, preferably varying periodically from 5 to 30 volume %.
According to a further feature of the invention, the molybdenum sulfide layer contains successive individual layers of different compositions of which one type comprises the mixture of MoS2 and Mo2S3 sulfide phases and the other type consists substantially of pure MoS2. Each of the MoS2 layers has a maximum thickness of 200 nm. The thicknesses of layers of the mentioned phase mixture (MoS2/Mo2S3) respectively lie between 200 nm and 400 nm.
In a further embodiment of the tool of the invention there are still further metallic phases and/or hard material phases in the molybdenum sulfide layer under consideration and whose volume proportion however is preferably not in excess of 10 volume %. The metals in question can be especially molybdenum and the materials especially molybdenum carbide, titanium carbonitride or chromium carbide. Such metal phases or hard material phases serve to increase the stability of this layer which however is in substance the molybdenum sulfide of the mixture described at the outset. The volume proportion of MoS2 in this layer lies between 30 and 98 volume %, preferably between 40 to 95 volume %. The layer thickness lies between 0.5 to 8 xcexcm, preferably between 2 and 4 xcexcm.
Optionally the single or outermost layer of molybdenum sulfide can have still a further metallic layer of a thickness xe2x89xa6200 nm applied thereto. Preferably metals of groups IVa to VIa of the Periodic Table are used. Alternatively these thin outer layers of a maximum of 200 nm in thickness can also be composed of a hard material, preferably a carbide, a nitride or a carbonitride of one of the aforementioned elements.
The molybdenum sulfide layer is applied by means of a CVD process, preferably using the subsequently described process technique.
For producing the mentioned coating, a CVD process is used in which the gas mixture is composed of molybdenum chloride, hydrogen sulfide, a noble gas and hydrogen and with a coating temperature lying between 500xc2x0 C. and 900xc2x0 C., preferably between 600xc2x0 C. and 850xc2x0 C. The pressure amounts to 0.1 to 50 kPa, preferably between 0.1 and 1 kPa. The molar ratio between hydrogen sulfide and molybdenum chloride lies between 0.1 and 4, preferably between 0.1 and 2. The proportion of hydrogen and argon together makes up between 80 to 99 volume %, preferably between 90 to 98 volume %.
Preferably MoCl5 is used although optionally MoCl4 or mixtures of MoCl5 and MoCl4 can be used.
The maintenance of the above-mentioned parameters is significant for the production of the layer consisting essentially of molybdenum sulfide. With greater than the aforedescribed molar ratios, layers are obtained which consist exclusively or predominantly of hexagonal MoS2. MoCl5 can decompose at temperatures above 200xc2x0 into MoCl4 and Cl so that the molybdenum sulfide can be formed both from MoCl5 and also from MoCl4.
The mixed layer, which contains hexagonal MoS2 and Mo2S3 can on the one hand be produced as a coating with a constant Mo2S3 proportion and random orientation of the MoS2 crystallites. On the other hand the method of the invention can also produce multilayer coatings which can have alternating layers of MoS2/Mo2S3 phase mixtures and layers of oriented MoS2. In the last-mentioned case, the MoS2 layers however should not be thicker than 200 nm since otherwise pores will develop in the molybdenum sulfide layer whereby the density thereof will diminish. In a special embodiment, the molar ratio H2S/MoCl5 during the coating alternates for equally long time periods in which one of the selected mole ratios is greater than the other by a factor of 7 to 10 so that with a greater mole ratio, pure MoS2 is deposited and with the smaller mole ratio, a phase mixture of MoS2 and Mo2S3 is deposited. The duration over which the substantially pure MoS2 is deposited is that which will provide a maximum permissible layer thickness of 200 nm. By comparison, the duration in which the mole ratio is set to deposit the phase mixture MoS2/Mo2S3 can be two or three times as great. Preferably 8 to 12 layers are deposited of which substantially half are composed of the mentioned phase mixture and the other half are of MoS2.
Alternatively here too, the molar.ratio H2S/MoCl5 can be so controlled that during deposition the Mo2S3 phase proportion periodically varies.
The molybdenum sulfide layer is composed in essence of hexagonal MoS2 and Mo2S3 although it can however contain still further phases which can be formed from the gas mixture of MoCl5, H2S, H2 and argon. In this manner, small proportions of rhombo-hedral MoS2, MoS2Cl2 and MoCl3 can arise which, however, have no noticeable effect on the layer characteristics.
The layer consisting essentially of molybdenum sulfide can also be formed in a multilayer manner whereby the individual layers containing essentially MoS2 and Mo2S3 can be separated by intermediate layers of a metal or a metallic hard material. The intermediate layers are however always thinner than 0.3 xcexcm. As materials for such intermediate layers, molybdenum or molybdenum carbide are to be considered, these enhancing mechanical stability of the molybdenum sulfide layer according to the invention. To deposit molybdenum or molybdenum carbide, the gas mixture composition in the CVD process is so altered that the feed of H2S is interrupted and replaced by CH4. By rapid switching of the gas it is possible to obtain quasicontinuous layers which aside from MoS2 and Mo2S3 also contain other phases. With this feature, the stability of the dry lubricant layer can also be improved. In practice one balances which components of the stability enhancing nonsulphidic impurities are present or intermediate layers are used. The lowest friction coefficients are obtained with layers which are comprised of hexagonal MoS2 but can have the stability by the additional Mo2S3 phase and further by nonsulphidic impurities. The choice of the respective proportions of stabilizing-enhancing compounds and/or metals of hard materials depends upon the kind of workpiece material to be machined and the cutting forces arising during machining. The invention is described in further detail using concrete examples: